This invention relates to a process for forming styrenic polymer beads which have anti-lumping properties upon pre-expansion and which are useable for producing soft molded products such as packaging material.
The formation of styrene polymer beads is a well known and commercial process. One such process is described in Grim patent, U.S. Pat. No. 2,673,194, the contents of which are incorporated by reference herein. Such styrene polymer beads, upon impregnation with a blowing agent, are expandable to discrete foam particles which can be formed into shaped articles by further expansion and then fusion thereof in contact with steam within the confines of a mold.
A useful property of expanded polystyrene articles is the cushioning effect that such articles provide when used as a packaging material. Attempts have been made to increase, even more, the cushioning properties of polystyrene articles, by enhancing the impact strength and resiliency of the expanded polystyrene used to form such articles. Such attempts include the polymerization of 60-70 parts of styrene within 40-30 parts of pelletized copolymers of ethylene and vinyl acetate. These particles, after impregnation with butane, yield molded foams that are softer, more resilient, and have greater resistance to impact than polystyrene foams. However, because of the fugitivity of the blowing agent, the product must be expanded promptly after its production or stored in pressure-resistant containers at low temperatures.
Polyethylene foams of a density of about two pounds per cubic foot exhibit good impact strength and resiliency but are deficient for some packaging applications because of a tendency to creep and form an enlarged cavity while carrying a relatively light load.
A problem relative to pre-expansion of expandable polystyrene beads before introduction into a mold is the tendency of the pre-expanded particles to clump together and form lumps which render them unsuitable for molding. These lumps cannot be properly conveyed in processing plants and are unsuitable for charging into molds of intricate contour due to improper fill-out of the molded articles. Even if reasonable fill-out is achieved, variations in density within the molded article can result and voids also occur.
Various methods have been proposed for the prevention of lumping during pre-expansion, but these have all suffered from some objectionable feature such as rapid loss of blowing agent, poor flow of the treated beads, dust problems which cause plugging of the steam ports, and often serious reduction in fusion of the particles on molding.
An object of the present invention is to produce expandable styrenic polymer beads which can be packaged, transported, and stored like conventional expandable polystyrene, and which will be usable in forming soft molded articles.
Another object of the present invention is to produce expandable polymer beads which are resistant to lumping upon pre-expansion of the beads.
A further object of the present invention is to provide a process which will produce coated polymer beads which have a shelf life sufficient to permit shipment in containers for conventional expandable polystyrene beads, and will be resistant to lumping upon pre-expansion of the same and will also be useable in the formation of soft molded articles.